Toxoplasma gondii establishes an important chronic infection capable of causing life-threatening toxoplasmic encephalitis in AIDS patients and other immunocompromised individuals. The basis of persistence of the infection is the tissue cyst, which remains largely quiescent for the life of the host, but can reactivate and cause disease. This stage of the parasite is not affected by any of the current drug treatments and it has been generally regarded as untouchable. However, our recent studies revealed that the immune T cells are able to eliminate T. gondii cysts from the brains of infected hosts when the T cells are transferred into infected immunodeficient animals that have already developed large numbers of the cysts. This T cell-mediated immune process is associated with accumulation of mononuclear cells, primarily microglia and macrophages morphologically, around tissue cysts. Since the accumulated phagocytes penetrate within the cyst, these cells appear to be the main effector cells that destroy the cysts and eliminate them from the brain after initiation of this process by immune T cells. CD8+ immune T cells possess a potent activity to initiate this anti-cyst immune process, and the protective activity of the T cells requires perforin. The overall goal of this research project is to determine the molecular mechanisms by which perforin-mediated activity of CD8+ immune T cells cooperates with the phagocytes to remove T. gondii cysts from the brain. In the first aim, we will determine if CD8+ immune T cells accumulate and attach to the cyst-containing cells in the brain. We will also determine if the CD8+ T cells lyse cyst-containing cells in a perforin-dependent manner in vitro and whether the perforin- mediated activity of CD8+ T cells induces the accumulation of phagocytes around the cysts in the brain. In the second aim, we will determine the MHC class I molecule(s) critical for recognition of cyst-containing cells by CD8+ T cells. We will then determine the T. gondii cyst epitope(s) presented by the identified MHC class I molecule for recognition by the CD8+ T cells by screening predicted bradyzoite epitope peptides and/or bradyzoite cDNA library. The third aim is to determine the mechanisms by which phagocytes eliminate T. gondii cysts after initiation of the immune process by CD8+ T cells. We will examine the roles of inducible nitric oxide synthase and autophagy in killing of bradyzoites and elimination of cysts using mice lacking a molecule required for each mechanism. The information generated from the studies in these three specific aims all together will provide the essential basis for developing a novel method(s) to activate CD8+ T cells and phagocytes to eliminate T. gondii cysts that can reactivate and cause toxoplasmic encephalitis. PUBLIC HEALTH RELEVANCE: Reactivation of chronic infection with Toxoplasma gondii causes life-threatening toxoplasmic encephalitis in AIDS patients and other immunocompromised individuals. The proposed studies are to analyze how T cells and phagocytes cooperate to eliminate the persisting cyst stage of T. gondii, which can reactivate and cause the disease. These studies will provide the information essential for developing a novel method(s) to eliminate the persisting cyst stage of the parasite and prevent or reduce the risk of developing toxoplasmic encephalitis.